1. Field of the Invention
The present invention relates to a method of and an apparatus for projecting a laser beam into a light amplifier and, more particularly, to a method of and an apparatus for projecting a laser beam into a light amplifier of a so-called broad contact type in which a stripe width of an active region is equal to or greater than a predetermined value.
2. Description of the Related Art
Recently, an examination has been conducted on the use of a laser diode as a light amplifier after making the mirror surfaces of the laser diode non-reflecting. As shown in FIGS. 2(A) and 2(B), the light amplifier is arranged such that an active region 14 is held between a n-type semiconductor region 10 and a p-type semiconductor region 12, and an anode 16 with a stripe width T is provided on the n-type semiconductor region 10, while a cathode 18 is provided on the p-type semiconductor region 12. In addition, mirror surfaces 20, 22 are coated so as to be non-reflective. A laser beam generated by a laser diode 26 is projected into the light amplifier 24 via a collimator 27, a light isolator 28 and a condenser 29, as shown in FIG. 3. If a bias is applied to this light amplifier in the forward direction, a positive hole and an electron flow, and luminescence and recoupling take place in the active region 14. In this state, when incident ray Pin is projected into the active region 14 from a mirror surface 20 side, amplified emission ray Pout is emitted from a mirror surface 22 side. In other words, although the semiconductor absorbs the incident ray Pin in the normal state of thermal equilibrium, when an injector amount of the positive hole and electron is large, and energy distribution of the positive hole and electron in the active region 14 becomes a state of so-called inverted distribution, the incident ray is not absorbed and is amplified while being propagated through a light waveguide region of the active region 14, and the ray is then emitted in the form of emission ray Pout through stimulated emission. At that time, a horizontal transverse mode which is parallel with the active region 14 is controlled by a gain waveguide mechanism or a refractive index waveguide mechanism. The gain waveguide mechanism makes use of a phenomenon in which, when there is no refractive index distribution, light is amplified by stimulated emission along a portion where the gain is high. Specifically, the light is confined in a region where the gain is high, and this region serves as a waveguide region. Meanwhile, the refractive index waveguide mechanism makes use of a phenomenon in which light is reflected in a boundary where a difference in the refractive index exists, a high refractive index portion is provided in the active region, and the high refractive index portion serves as a waveguide region. When light is introduced into the waveguide region, total reflection occurs repeatedly on the boundary surface of the high refractive index portion and is propagated. A normalized frequency V of the above-described light amplifier and a refractive index B of a normalized waveguide region, which shows a degree of confinement of light, have relationships shown in FIG. 4. It should be noted that a mark M in the drawing denotes a degree of the horizontal transverse mode. As can be understood from FIG. 4, the higher the normalized frequency V, the more the light propagated through the guiding region becomes a multimode.
In the case of a broad contact type of a light amplifier in which the width of the waveguide region, hence, the stripe width, is equal to or greater than a predetermined value (10 .mu.m), it is possible to obtain a high output laser beam of several hundred milli watts to one watt. However, there is a problem in that, since a restriction in a lateral transverse direction is alleviated due to the large stripe width and the horizontal transverse mode has become a multi-mode (see FIG. 3), it is impossible to allow the light to converge into a spot of several microns depending on an optical system. In other words, if consideration is given to a light amplifier or a refractive index waveguide type amplifier which makes use of the refractive index waveguide mechanism, the normalized frequency V is expressed by the following formula (1), so that, if the stripe width T becomes large, the normalized frequency V becomes high. Consequently, as can be seen from FIG. 4, the laser beam propagated through the waveguide region assumes a multi-mode, so that it becomes impossible to allow the laser beam to converge up to a diffraction limit in the Gaussian distribution: EQU V=k.sub.0 T.sqroot.n.sub.1.sup.2 -n.sub.2.sup.2 (1)
where, k.sub.0 is a constant, n.sub.1 is a refractive index of a waveguide region, n.sub.2 is a refractive index of a portion of the active region which is adjacent to the waveguide region.
To solve the above-described problem, the stripe width may be narrowed or n.sub.1.sup.2 -n.sub.2.sup.2 may be made small so as to be lower the normalized frequency V, thereby establishing a single lateral transVerse mode. However, if n.sub.1.sup.2 -n.sub.2.sup.2 is made small, a difference in the refractive indexes becomes small, and therefore the light becomes liable to be transmitted from the waveguide region to the active region, so that the degree of confinement of light drops. On the other hand, if the stripe width is narrowed, the photon density in the waveguide region becomes high, with the result that optical damage results.
In addition, the same also applies to a light amplifier which makes use of the gain waveguide mechanism or a gain waveguide type light amplifier, since the magnitude of the gain corresponds to the magnitude of the refractive index.
In the light amplifier of a single mode, the above-described multi-mode does occur. However, there are limits to obtaining a high output, so that it is impossible to obtain a very large output.
In addition, in a technique disclosed in "Progress in Semiconductor Laser Diodes (1986)", SPIE, Vol. 723, pp. 36-39, a technique is disclosed for obtaining a high output using a light amplifier having a large stripe width. However, since a slit disposed between two cylindrical lenses is used as a light isolator, part of a laser beam is shielded at an end portion of the slit, so that the incident laser beam which is projected into the light amplifier becomes a multi-mode. For this reason, this arrangement is unsuitable in a case where a high degree of resolution is required and a laser beam is condensed into a very narrow range with high power.